Time-Varying Dynamics of a Tiltrotor during its Transition Phase

In this paper, the time-varying dynamics of a tiltrotor during its transition from the helicopter mode to the fixed wing aircraft mode, are investigated from a flight dynamics perspective. The time scales over which the nacelle angle variation occurs are considered to be of the same order as the flight dynamics of the tiltrotor, due to which the time-varying aspects of the dynamics must be carefully evaluated. A characterization of the influence of different time trajectories of the nacelle angle on the flight dynamics, is performed. The concept of a time-varying pole is adopted and the flight dynamic properties of the time-varying modes are determined. This time-varying pole is used to determine time-varying short period and phugoid modes, which are generalizations of the conventional short period and phugoid modes. A tool to properly evaluate the sensitivity of these time-varying modes to different nacelle angle trajectories is demonstrated, along with the degree of controllability of the tiltrotor's flight dynamics for each such trajectory.